A. Field of the Invention
This invention relates to solid gas generants particularly those suitable for the production of substantially pure nitrogen gas, and more particularly to the use of non-azide gas generant compositions that are characterized by their freedom, when burned, from the formation of toxic species such as hydrogen cyanide, nitrogen oxides and carbon monoxide.
B. Description of the Prior Art
There has been considerable interest in the generation of nitrogen gas for a number of purposes including the inflation of aircraft or automobile safety crash bags, also termed air bags.
The details of the crash bag systems have been widely discussed as have the reasons for the selection of pyrotechnic devices for the generation, rapidly and dependably, of the gas for inflating the bag. The operational constraints of crash bags are also well known. The system must supply absolutely nontoxic gas to inflate the bag because crash bag systems generally vent into the passenger compartment on deflation, and because of the very real probability of bag rupture in an actual crash situation. Additionally, the gas must inflate the bag at a temperature which the vehicle occupant can tolerate. The time period for attainment of maximum inflation has been determined to be from 20 to 100 milliseconds. The device must be safe to handle and store prior to production. It must be adaptable to mass production line installation techniques and not introduce an unreasonable hazard then or during the life of the vehicle. It must assure reliable operation during the life of the vehicle containing it, which may be 10 years or longer.
The objectives of rapid generation of cool non-toxic inflation gas and long-term operability depend to a large extent on the gas generant selected and the physical form into which it is initially compounded.
If a suitable propellant can be designed, then the design of a complete passive restraint system undertaken with consideration of the characteristics of a particular propellant stands a better chance of practical success.
Naturally, from every point of view, the most desirable atmosphere inside an inflated crash bag would correspond in composition to the air outside it. This has thus far proven impractical of attainment. The next best solution is inflation with a physiologically inert or at least innocuous gas. The most practical of these gases has proven to be nitrogen.
The most successful of the prior art solid gas generants of nitrogen that are capable of sustained combustion have been based upon the decomposition of compounds of alkali metal, alkaline earth metal and aluminum derivatives of hydrazoic acid, especially sodium azide. Such a nitrogen gas generant comprising mixtures of alkali metal azides, metal and metalloid oxides, molybdenum disulfide, and optionally sulfur, pressed into pellets is disclosed in U.S. Pat. No. 4,203,787 that was granted on May 20, 1980 to George F. Kirchoff and Fred E. Schneiter.
There are disadvantages, however, to the use of these azides, particularly in the generation of the inflating gas for air bag systems.
Sodium azide, a Class B explosive, is a highly toxic material. It is easily hydrolyzed, forming hydrazoic acid which is not only a highly toxic and extremely sensitive explosive gas, but it also readily reacts with heavy metals such as copper, lead, etc. to form extremely sensitive solids that are subject to unexpected ignition or detonation. Especial handling in the manufacture, storage and eventual disposal is required to safely handle such materials and gas generants prepared from the azide compounds.
A number of approaches to a non-azide nitrogen gas generant have been investigated in the prior art, as disclosed, for example, in U.S. Pat. Nos. 3,004,959, 3,055,911, 3,171,249, 3,719,604 and 3,873,477. Many of the prior art nitrogen gas generants that have been reported are based upon nitrogen-containing compounds such as those derived from the various hydroxamine acid and hydroxylamine derivatives, while others consist of various polymeric binders, hydrocarbons and carbohydrates which are oxidized to produce non-corrosive and, often termed, "non-toxic" gases. The gas products from these compositions contain unacceptably high levels of carbon dioxide, carbon monoxide and water for use in crash bag applications where the possibility exists that the occupant may breathe, even for short periods of time, high concentrations of the gases produced from the gas generant. These compositions do not meet the present requirements that the combustion products meet industrial standards for toxic and other gases such as carbon monoxide, carbon dioxide, etc.
Other approaches to a non-azide nitrogen gas generant consist of using tetrazole compounds such as aminotetrazole, metal salts of aminotetrazole, or other tetrazole salts which contain hydrogen in the molecule with oxygen containing oxidizers such as potassium perchlorate. These mixtures, when burned, tend to form various toxic species such as hydrogen cyanide, nitrogen oxides, and carbon monoxide, and hence, also do not meet the present requirements in respect of the combustion products that are produced.